In recent years, lithium secondary batteries, which have a higher discharge voltage and a higher energy density than conventional small aqueous secondary batteries, have been put into practical use as the main power sources for portable appliances. When lithium metal is used as a negative electrode active material, a lithium secondary battery having a high energy density can be obtained because lithium metal has a high electrochemical capacity per weight.
However, during charging of the battery having lithium metal in the negative electrode, dendrite deposits on the surface of the lithium metal, which grows during repeated charge/discharge cycles. When the grown dendrite comes into contact with the positive electrode, a short circuit occurs inside the battery.
Moreover, the dendrite is highly reactive because of its large surface area, so that it reacts with an organic solvent in the electrolyte. Consequently, a coating film having a low electronic conductivity, which is made of decomposition products of the solvent, is formed on the surface of the dendrite, thereby increasing the internal resistance of the battery.
Further, on the surface of the negative electrode, the grown dendrite often causes lithium particles isolated from the conductive network, thereby decreasing the charge/discharge efficiency of the battery.
Therefore, a lithium secondary battery using lithium metal as a negative electrode active material has problems of a low reliability and a short cycle life.
The materials that are currently studied as negative electrode active materials alternative to lithium metal include: a carbon material capable of absorbing and desorbing lithium; a lithium-containing metal oxide such as Nb2O5 or Li(Li1/3Ti5/3)O4 (Japanese Laid-Open Patent Publication No. 2000-12090); a lithium-containing metal nitride such as Li7MnN4, Li3FeN2 or Li2.4Co0.6N (Japanese Laid-Open Patent Publication No. hei 9-35714); and an intermetallic compound, a solid solution or an oxide each containing Si, Sn or the like as a constituent element (Japanese Laid-Open Patent Publication No. hei 11-86854).
When a carbon material is used in the negative electrode, the charge reaction is the absorption reaction of Li ion into the carbon material. Accordingly, the deposition of lithium does not occur and, therefore, problems due to the dendrite do not occur. Similarly, when an intermetallic compound, a solid solution or an oxide each containing Si, Sn or the like as a constituting element and having a discharge capacity of approximately 2000 mAh/g is used, the formation of the dendrite does not occur.
Among practically used carbon materials, the theoretical capacity of, for example, graphite is 372 mAh/g. This is only about 10% of the theoretical capacity of lithium metal. Additionally, the capacity of a lithium-containing metal oxide such as Nb2O5 or Li(Li1/3Ti5/3)O4 is 100 to 200 mAh/g, which is even smaller than those of carbon materials.
The capacity of a lithium-containing metal nitride such as Li2.4Co0.6N is approximately 760 mAh/g. Therefore, a battery using the lithium-containing metal nitride in the negative electrode has a higher energy density than the one using a carbon material or a lithium-containing metal oxide. However, the lithium-containing metal nitride is inferior to conventional carbon materials in terms of the cycle characteristics of the battery.
Similarly, a battery which uses in the negative electrode an intermetallic compound, a solid solution or an oxide each containing Si, Sn or the like as a constituting element has a problem of a short cycle life. This problem is attributable to the fact that the redox reaction of the battery comprises the reaction wherein an alloy of lithium and the negative electrode active material is formed, and the reaction wherein Li is desorbed from the alloy, so that the rate of the expansion and contraction of the negative electrode active material during charging and discharging is high. Accordingly, during the repetition of the charge/discharge cycle, the negative electrode active material is pulverized owing to the internal stress, thereby destructing the conductive network.
The purpose of the present invention is to solve the above-discussed problem and to provide a lithium secondary battery having a high energy density and a long cycle life, and more specifically, to provide a negative electrode active material used therein.
The present invention relates to a lithium secondary battery comprising a positive electrode capable of absorbing and desorbing lithium, a non-aqueous electrolyte and a negative electrode capable of absorbing and desorbing lithium, wherein the negative electrode comprises a nitride represented by the general formula: LixAyMezN, where A is boron, silicon or aluminum, Me is at least one element selected from the group consisting of transition metal elements and metal elements of Group IIIB, IVB and VB, and x, y and z satisfy 0 less than x less than 3, 0 less than yxe2x89xa61, 0 less than zxe2x89xa61 and 0 less than x+y+zxe2x89xa63.
The Me is preferably at least one element selected from the group consisting of Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zr, Nb, Mo, Ru, Sn, W, Pb and Bi.
While the novel features of the invention are set forth particularly in the appended claims, the invention, both as to organization and content, will be better understood and appreciated, along with other objects and features thereof, from the following detailed description taken in conjunction with the drawings.